dual boot mac startup key

When you installed the system did you check the box to install the OS 9 drivers when initializing the disk?
The "Startup Disk" system preference should show both OSX and OS 9 if they are both on the disk. Does Classic work?

There is no boot key to select OS9, but if the system is set to boot OS 9, you can hold down "X" to make it boot OSX.

Hi, Robert -

Startup Manager, invoked via holding the Option key at startup, can display only one OS per volume - and will display the last OS booted to on that volume.
Article #106178 - Startup Manager: How to Select a Startup Volume

You should be able to use the Startup Disk control pane in System Preferences to set OS 9 as the boot OS, then restart into it.

This is, of course, assuming your machine is OS 9 bootable. Not all G4 MDDs are.

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If you do want to be able to use Startup Manager to pick between the two, you will need to get the two OS's onto separate volumes, either separate partitions or separate drives.